Basicity electronegativity effects

Comparison of the intrinsic acidities and basicities of pyrrole (I), imidazole (4), and pyrazole (6), together with complementary information coming from the azine held, illustrate the main effects that control the acidity and the basicity of unsubstituted azoles (86JA3237). Particularly important are the role of electrostatic interactions between adjacent charged nitrogens (NH) and between adjacent lone pairs (N), as well as the aza electronegative effects. 

The hypothesized delocalization of lone pair electrons in the above silicon compounds is supported by the lowered basicity of the silyl compounds as compared to the corresponding carbon compounds. This reduced basicity is contrary to that expected on the basis of electronegativity effects operating through the a system since silicon is less electronegative than carbon. It is consistent with an internal Lewis acid-base interaction between the nitrogen and oxygen lone pairs and empty acceptor d orbitals on the silicon. Experimentally this reduced basicity is shown by the absence of disiioxane adducts with BF3 and BO ... 

The strong electronegativity of the fluorinated substituents is reflected in the effect that this group has upon the acidity of alcohols and carboxylic acids, as well as the effect it has on the basicity of amines (Tables 1.3-1.5). 

Oxygen is so electronegative that inductive effects from substituents have rather less influence on basicity than they would in similar nitrogen compounds. Alcohols are somewhat less basic than water, with ethers weaker still. 

Deprotonation of 9 and 10 illustrates this effect in the absence of TMEDA, deprotonation occurs ortho to the more Lewis-basic amino group, while in the presence of TMEDA, the deprotonation occurs ortho to the more electronegative, and therefore more acidifying, MeO group (Scheme 6). ... 

The main conclusions of this study are (86JA3237) (1) that pyrazole (6) is less basic than imidazole (4) (see Table V) is mainly due to the electrostatic repulsion NH NH in the pyrazolium cation (6H ) (2) that 1,2,4-triazole (8a) is less basic than imidazole (4) (see Table V) is mainly due to the electronegative aza effect (3) that imidazole (4) is more acidic than pyrazole (6) in the gas-phase (see Table VI) is a consequence of the lone pair/lone pair electrostatic interaction in the pyrazole anion (6 ). 

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